The effects of pipe diameter size and surfactant additive on the two-phase gas/liquid flows regimes, pressure gradients, bubble sizes and velocities were investigated. Experiment were carried out using three vertical transparent tubes with inner diameters of 10.75, 19, and 53.15 mm and the length of 3 m. Water, octylbenzyldimethylammonium chloride solutions (C8H17-C9H13NCI) and hexadecyl-benzyldimethylammonium chloride solution (C16H33-C9H13NCI) were used as the working fluids. The boundaries of the flow regimes for a given Rewater increase nonlinearly with increasing tube diameter. Adding the surfactant lowers the critical air Reynolds numbers for the bubble-slug flow and the slug flow regimes. The friction factors or the dimensionless pressure gradients are distinctively different depending on diameter and type of liquids used (water, (C8H17-C9H13NCI) and (C16H33-C9H13NCI)). The normalized bubble or slug dimension for pipe diameter of 10.75 mm is always greater than those of the pipe diameter of 19 mm and 53.15 mm at any Reair and Rewater. The normalized bubble or slug dimension for pure water is greater than those of (C8H17-C9H13NCI) solution and (C16H33-C9H13HCI) solution. The normalized bubble and slug velocities of pipe diameter 10.75 are always greater than those of pipe diameters of 19 mm and 53.15 mm. The normalized bubble and slug velocity for pure water is lower than those of (C8H17-C9H13NCI) solution and (C16H33-C9H13NCI) solution due to surface tension effect.